Blast load calc for a cylindrical tower with lateral restraint at top

[fizzy]

Hi All,

Can someone please throw more light on this or refer me to a text to help me clarify this with my client.

i'm designing a vertical tower to the EN13445 code with severe blast loading on its supports resulting in an unrealistic thickness compared to the vessel. This is also not giving me an acceptable calculation. My client has stated there will be a lateral restaint at the top which i believe will have an impact on the maximum bending moment which seems to be the determining factor for skirt and base ring design.

I'm basically applying the blast pressure by multiplying the vessel area and considering this as a uniform load acting on the vessel.(typical to wind loading calculation on vessels). Since vertical vessels under wind loadings are designed as cantilever with uniformly distributed load and the maximum bending moment at the base ring is taken as Mmax = ?l2/2(maximum bending load for a cantilever beam with uniformly distributed load). The maximum bending moment for my case with lateral restraint should be ?l2/8 (maximum bending moment for a fixed beam at one end, supported at the other end and uniformly distributed load).

From the above, the eqn with a restraint reveals to have a quarter less the maximum bending moment as oppose to an un-restrained vessel.(A typical vessel supported on just a skirt).

Am i right in working backwards and considering a quarter the blast pressure for my design.

If yes can you pls refer me to a text.

I look forward to your replies...

Many Thanks in advance.
fizzy

 

[JStephen]

I don't know why you'd want to consider a quarter of the blast pressure, just figure the blast pressure and the equation you noted for the moment.

You could potentially look at dynamic effects as another way to reduce the design load.

 

[fizzy]

Thanks JStephen for your feedback.

The reason i'm considering a fraction of the blast pressure is because, i'll have to input a blast pressure for the skirt support calculation as the code or my software assumes the vessel to be self supported which isn't the case in my scenario.

With the determination of the max bending moment at the base ring, isn't it logical to work backwards to determined the blast pressure resulting in that max bending moment.

 

[trottiey]

You approach seems reasonable. You could baffle your customer with St. Venant's Principle if he's skeptical.

 

[JStephen]

With a cantilever, maximum moment is at the base, with a supported beam, maximum moment is away from the supports, so it's not just the same as adjusting the moment.